Biochar-induced Oxidation Of As(?) Or Reduction Of Cr(?) And Their Subsequent Immobilizations In Water:Effects Of Environmental Factors

In several decades,biochar has received increasing attention on environmental remediations.In this dissertation,we summarized biochar sources,compositions,structural properties,and their structure-application relationships.Subsequently,we reviewed the advances in As(?)oxidation and Cr(?)reduction,and their immobilizations by biochar,especially potential effects of environmental factors on As(?)oxidation and Cr(?)reduction and their subsequent immobilizations.To date,however,the mechanistic effects of environmental factors involving solution pH,dissolved O2,environmentally coexisting substances(i.e.,extrinsic Ca(?)and Fe(?)),and/or intrinsic iron oxides on As(?)oxidation or Cr(?)reduction and their immobilizations still remain poorly understood.Herein we systematically investigated the mechanistic effects of the environmental factors on As(?)oxidation or Cr(?)reduction and their subsequent immobilizations by rice-husk-derived biochar,the main contents and results of the dissertation were summarized as follows:(1)Here we investigated effects of solution pH(3.0,7.0,and 9.5)and dissolved O2(anoxic and oxic conditions)on As(?)oxidation by biochar.The results showed that the promoted oxidation of As(?)by biochar was strongly pH and O2 dependent.Under anoxic conditions,no As(?)oxidation by biochar occurred at pH 3.0 and 7.0,while As(?)was thoroughly oxidized by biochar within 12 h(2.161×10–1 h–1)at pH 9.5.More interestingly,the promoted As(?)oxidation by biochar was observed at all the pH values under oxic conditions(with 1.402×10–1 h–1at pH 3.0,0.513×10–1 h–1 at pH 7.0,and 3.817×10–1 h–1 at pH 9.5),while O2 alone slightly oxidized As(?)at pH 9.5.Under acidic and neutral pH conditions,only the dominant oxidation of As(?)by ·OH produced from activation of O2 by phenolic –OH and semiquinone-type PFRs occurred.In contrast,the oxidation by both semiquinone-type PFRs and quinoid C=O and H2O2(if O2 was introduced)appeared under alkaline conditions.This pH-dependent oxidation mode was mainly attributed to the varying redox activities of redox-active moieties(RAMs,i.e.,phenolic –OH and semiquinone-type PFRs,and quinoid C=O),as confirmed by multiple characterization and validation experiments using biochar with tuned RAMs compositions,as well as thermodynamics.(2)Oxidation and immobilization of As(?)by reductively undissolved(r UBC)and dissolved(r DBC)fractions of biochar were investigated in the presence of Ca(?)under alkaline and anoxic conditions.The results demonstrated that the co-presence of Ca(?)and r UBC synergistically oxidized and immobilized As(?).r UBC completely oxidized As(?)within 8 h,but failed to immobilize As(?)and newly produced As(V),while Ca(?)alone neither oxidized nor immobilized As(?).Compared with r UBC,r DBC led partly to the As(?)oxidation,but could not immobilize As(?)and As(V).The presence of Ca(?)significantly enhanced the As(?)immobilization by r DBC,but had a detriment effect on the As(?)oxidation.Multiple characterization and chemical precipitation model were applied to decipher the interesting phenomena.In the r UBC+Ca(?)+As(?)system,As(?)was fully oxidized to As(V)by RAMs(i.e.,PFRs and quinoid C=O)on r UBC,and followed by precipitation with the adsorbed Ca(?)on r UBC.In the r DBC+Ca(?)+As(?)system,however,r DBC induced local enrichment of Ca(?)in the nascent flocs for predominant non-oxidative and slight oxidative precipitation of As(?),via ternary r DBC–Ca(?)–As(?/V)complexation.In addition,both the increase in pH(10.0–12.5)and Ca(?)concentration(0–100 m M)were generally conducive to oxidation and immobilization of As(?)by r UBC and r DBC.(3)Fate and oxidation of As(?)in nascently oxic water was explored in the presence of biochar and Fe(?)under circumneutral pH conditions.The results showed that As(?)was partly oxidized to As(V)by biochar,but no As(?)and newly produced As(V)immobilization occurred over the whole experimental period of 21 d.At the oxidationprecipitation stage(1 d),the Fe(?)oxidative precipitation induced that As(?)concentration in aqueous solution decreased quickly from 100 to 19.8 ?M within 3 h,and to 0.1 ?M afterward,while the newly produced As(V)concentration in aqueous solution increased from 0 to 5.2 ?M,but subsequently decreased to 0.The presence of biochar facilitated oxidative precipitation of Fe(?)and As(?)(i.e.,As(?)concentration in aqueous solution decreased from 100 to 3.1 ?M within 3 h,and to 0.5 ?M,while the newly produced As(V)concentration in aqueous solution elevated from 0 to 11.0 ?M,but subsequently decreased to 0.8 ?M),but biochar itself did not oxidize As(?).At the aging stage(1–21 d),the total As(i.e.,mostly As(?))concentration in aqueous solution in the Fe(?)+As(?)system increased from 0.1 to 4.6 ?M due to the transformation of arsenic-containing lepidocrocite and amorphous iron mineral to goethite.In contrast,the total As concentration in aqueous solution(i.e.,mostly As(?))in the BC+Fe(?)+As(?)system elevated significantly from 1.3 to 12.0 ?M because of the decomposition of biochar,especially silicate release for competitive adsorption with solid-phase As(?),rather than transformation of these iron minerals.The presence of biochar dramatically inhibited the transformation of arseniccontaining lepidocrocite and amorphous ferric iron mineral to goethite,thus facilitating As(?)retention on the iron minerals.(4)Magnetic biochar(MBC)synthesized by microwave co-pyrolysis of solid-state Fe SO4 and rice husk was employed to remove Cr(?)from water.In comparison with the rice husk biochar(BC),the MBC exhibited the 3.2-and 11.7-fold higher adsorption and reduction efficiency of Cr(?).Multiple characterization results revealed that the high Cr(?)removal performance of MBC was mainly attributed to the presence of active Fe3O4 and carbon-centered PFRs in the porous and graphitic MBC.The Fe3O4 not only provided active chemisorption/reduction sites for Cr(?),but also facilitated the generation of more active electron donating carbon-centered PFRs than carbon-centered PFRs with an oxygen atom in the graphitic structure to reduce Cr(?).The presence of Fe3O4 also elevated BET-surface area and pore volume of MBC,promoting the Cr(?)removal.The Fe3O4 and carboncentered PFRs contributed to ?81.8% and ?18.2% of total Cr(?)generation,respectively.In addition,the initial solution pH was responsible for determining the relative significance of Cr(?)adsorption and reduction by MBC.